Illness behaviors, or affective, cognitive and behavioral responses to physical symptoms, account for a large proportion of U.S. healthcare costs, and are associated with clinically relevant outcomes, including symptom exacerbation or relief and functional recovery. Illness behaviors, in turn, are influenced by social circumstances above and beyond objective health indices; self-initiated contact with the medical system, one aspect of illness behavior, is partly determined by access to health-promoting resources or the help and advice received from social network members. Relationships are associated with health outcomes through multiple direct (e.g., behavioral norms, resources, physiology) and indirect (e.g., buffered stress, increased self- efficacy) pathways. Illness behavior deserves increased empirical attention as a key behavioral pathway linking interpersonal contexts to health, as it incorporates individual differences in medical care-seeking, as well as the cascade of responses that occur prior to care-seeking decisions. Evidence from observational and experimental research suggests illness behaviors are both genetically influenced and socialized within families. Studies of behavioral development, however, acknowledge the interdependence of genes and environment (i.e., people play a role in choosing or creating interpersonal contexts, or are differentially influenced by them, based on their genetic make-ups); thus, the joint mechanisms of genes and environment on illness behavior are not yet understood. The aims of the current proposal are to examine: 1) associations between childhood family environments and illness behavior development in early and late adulthood, 2) genetic and environmental contributions to illness behavior, and 3) proximal, interpersonal networks' associations with illness behavior and subsequent functional status in late adulthood. In addition to addressing genetic and social mechanisms of illness behavior development across the lifespan, this proposal will support the applicant's training aims of learning and applying new skills in longitudinal data analysis, biometrical analyses with twin and family-adoption designs, and theory on healthy aging and medical services in late-life. As chronic illness is the leading cause of death and disability worldwide, psychosocial issues in medical care, and the continuity of this care, are increasingly relevant to health promotion efforts. Developmental frameworks for health promotion acknowledge the importance of early life factors in pathways to late-life health. In particular, research suggests that social factors (e.g., modeling, social integration, support) across the lifespan are associated with a variety of illness behaviors and health outcomes (e.g., care-seeking, adherence, disability), above and beyond biological risks. The proposed research applies a genetically informative, longitudinal design to examine the social development of illness behaviors, both in early- and late-adulthood.